The present invention relates generally to switching power supplies and, more particularly, to a simplified control circuit for synchronous rectifiers used in ZVS phase shifted full bridge converters.
Recent advancements in MOSFET switch technology makes synchronous rectifiers a feasible and competitive rectification technique for low voltage, high current DC/DC converters. Synchronous MOSFET switches are used to increase efficiency, reduce power losses, reduce electromagnetic interference and facilitate zero voltage switching (ZVS) in such converters. However, proper timing of the synchronous rectifiers is critical to the efficient operation of the converter.
In one known technique, the synchronous rectifiers may be driven by the control circuit used to drive the primary side of the transformer in the converter. This technique is commonly referred to as control driven synchronous rectification. One proposed implementation of this technique is presented by Laszlo Balogh in xe2x80x9cDesign Review: 100W, 400 kHz, DC/DC Converter with Current Doubler Synchronous Rectification Achieves 92% Efficiencyxe2x80x9d, SEM-1100 edition of the Unitrode Switching Regulated Power Supply Design Seminar Manual, by Unitrode corporation, 1996.
In this proposed prior art technique, the DC/DC converter is implemented using a phase shifted full bridge configuration on the primary side and a current doubler output with synchronous rectification on the secondary side. Of particular interest, a secondary control circuit 10 as shown in FIG. 1 is used to drive the synchronous rectifiers (not shown). The secondary control circuit 10 receives as input the primary drive signals 12 for each of the switching devices on the primary side of the converter. The drive signals 12 are in turn transmitted via a signal transformer 14 to the secondary side of the converter. The secondary control circuit 10 then outputs a secondary drive signal 16 for each of the synchronous rectifiers.
Referring to FIG. 2, a timing diagram further illustrates the operation of the prior art secondary control circuit 10. In this proposed technique, both of the synchronous rectifiers are turned on during a dead time period of operation as shown at 22, where a dead period is defined as the time at which no current is flowing from the input voltage source through the primary winding of the transformer of the converter. When diagonal switching devices on the primary side of the transformer are conducting, one of the two synchronous rectifiers is generally in an on state. The onset of a dead time period occurs when one of the two diagonal conducting switching devices is driven to an off state. It should be noted that the second of the two synchronous rectifiers is driven to an on state at the same time as the diagonal switching device is driven to an off state. However, due to parasitic delays and other variations inherent in the switching devices, it is conceivable that the second synchronous rectifier may be driven to an on state before the diagonal conducting switching device is driven to an off state. In this scenario, an opportunity for cross conduction between the synchronous rectifiers is introduced into the converter.
Therefore, it is desirable to provide an improved control circuit for controlling the synchronous rectifiers employed in switching power converters. It is envisioned that the improved control circuit is operable to drive both synchronous rectifiers to an on state during a freewheeling period, and yet eliminate the risk of cross conduction between the synchronous rectifiers.
In accordance with the present invention, an improved secondary control circuit is provided for controlling synchronous rectifiers in a switching power converter. The secondary control circuit employs control signals available for driving primary full bridge switches to drive two synchronous rectifiers. In particular, the secondary control circuit is operable to drive both synchronous rectifiers to an on state during a dead time period of operation. The dead time period is defined as the time at which no current is flowing from the input voltage source through a primary winding of a primary transformer of the converter. The onset of the dead time period occurs when the diagonal conducting switching devices is driven to an off state. In other words, the secondary control circuit is further operable to drive the second of the two synchronous rectifiers to an on state only after one of the diagonal switching devices is driven to an off state by the primary control circuit. In this way, the improved secondary control circuit eliminates the risk of cross conduction between the synchronous rectifiers.
For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings.